The chemical conversion coating of aluminum, and its alloys, is known in the art as a process whereby the surface of the metal is chemically converted to a surface that more easily accepts applied coatings, i.e. paint, and increases the corrosion resistance of the metal. The corrosion resistance aspect of the art is of particular importance in the aircraft industry as they frequently employ aluminum alloys that are easily corroded.
An accepted process involves the use of chromates and or dichromates to produce a surface coating of acid based chromium. As these are rather toxic, and have a considerable environmental concern, the chromate based conversion coatings are being displaced by less toxic materials. Alternative conversion coatings which can be used are based on potassium permanganate and zirconium compounds, either by themselves or incorporated in a polymer system.
The general disadvantage of using a polyester coating as a coating for the aluminum and aluminum alloy surfaces is that the polyesters are generally easily hydrolyzed and/or broken down in wet humid environments.